A conventional driving apparatus of stepping motors is disclosed in the Japanese Patent Application non-examined publication No. H06-343294. FIG. 4 shows a block diagram of the conventional driving apparatus.
In FIG. 4, driving circuit 302 magnetizes sequentially the coils of two-phase stator of stepping motor 301, thereby driving motor 301 in a stepping manner. Up-down counter 351 counts an instruction signal, i.e. a stepping signal, which drives motor 301 in a stepping manner. Read-only-memories (ROM) 352 and 353 receive the count values by counter 351, and output prerecorded magnetizing-signal-data responsive to the counted value. Digital to analog converters (D/A converters) 354 and 355 receive the digital magnetizing signal data from ROMs 352 and 353, convert the data into analog voltage data, and finally output the analog data into driving circuit 302. Driving circuit 302 receives voltage signals from D/A converters 354 and 355, amplifies the voltage signal, and applies the resultant voltage to motor 301, whereby motor 301 can be driven as described above.
In the structure discussed above, the data stored in ROMs 352 and 53 are set to draw a pseudo-sine-wave so that a number of steps required rotating the motor one turn increases, thereby narrowing a step angle per step. As a result, the motor rotates smoothly.
However, in the conventional structure discussed above, irregular torque such as detent torque affects the operation whereby accurate positioning control of the motor cannot be expected. The motor produces toque in order to maintain the original position against outside torque that tries to rotate the rotor without magnetizing the stator coils. The maximum value of this torque is called detent torque.
When an encoder is coupled to motor 301 so that rotating information of motor 301 is fed back to a control circuit, i.e. closed-loop-positioning-control is provided, an accurate positioning can be realized; however, it requires a complicated circuit.